The present invention relates to eddy current imaging of defects in materials, and, more particularly, to using phase difference to determine the depth of defects.
In the prior art of FIG. 1, eddy current probes contain two coils 14a and 14b that are simultaneously excited from an oscillator 16 through a bridge circuit 10 having resistors 12a and 12b. The probe is disposed near a material to be examined for defects. The probe coils may be arranged so that both lie in a plane parallel to the material (differential geometry) or so that one lies near the material and the other serves as a reference (absolute geometry). Although only the differential-probe geometry is discussed here, the same principles apply to the absolute-probe geometry. The received signal, that is, the voltage across the coils, is mixed in mixer 30b with the excitation signal phase-shifted by 90 degrees to produce the quadrature (Q) signal component. Likewise, the received signal is mixed in mixer 30a with the excitation signal directly (i.e., phase-shifted by 0 degrees) to form the in-phase (I) signal component. The signals from mixers 30a and 30b are then respectively passed through filters 31a and 31b and then respectively through amplifiers 32a and 32b. Filters 31a and 31b may be low-pass (typical cut off frequency, 100 Hz), high-pass (typical cut off frequency, 5 Hz), or a combination of the two, i.e., a bandpass filter. The output signals from amplifiers 32a and 32b are then each applied to rotation circuits 33. Circuit 33a forms a signal H=I cos (.theta.)-Q sin (.theta.), while circuit 33b forms a signal V=I sin (.theta.)+Q cos (.theta.), wherein .theta. is a desired rotation angle represented by a DC input control voltage to circuits 33, which have ROM look up tables to generate sin .theta. and cos .theta., and H and V respectively stand for horizontal and vertical axis. Thus each of circuits 33 can additionally comprise a pair of multiplier circuits, each of which receives the I or Q signals and also the .theta. signal, with the outputs of the multipliers in respective rotation circuits 33 being added together.
The coils are scanned manually or mechanically over the material to be examined. In the vicinity of a defect, a change in the H and V signals occurs. One of these signal components (V in FIG. 1) is then passed through a threshold circuit 34, such as a differential amplifier, having a predetermined threshold amplitude as determined by a potentiometer 35, for defect detection. The signal from threshold circuit 34 can be applied to an alarm (not shown). Defect measurements are possible with this technique for a minimum defect length of about 10 to 15 mils (0.254 to 0.381 mm). By defect "length" is meant the larger of the two defect dimensions parallel to the scanned surface of a plate material (not shown in FIG. 1). If desired, circuit 33a can be eliminated.
With the prior art, defect depth (the dimension perpendicular to the scanned surface) cannot be directly measured. A means to quantitatively characterize defect depth makes it possible to distinguish between defects and small surface dents or scratches, thus determining the true material quality. Therefore a depth sizing technique is of immediate potential benefit.
It is therefore an object of the invention to determine the depth of surface-breaking defects.